In a process control environment a process control system controls a particular process, e.g., a continuously operating manufacturing plant. At different points, or stations, along the process, there is available a data element or variable which can be measured and/or controlled. These data elements may include pressure, flow, temperature, and calculated value which may include some of the aforementioned data elements. Within the process control system there is a need for information to be displayed at various points in the process. This display will allow an operator to make the necessary adjustments and/or decisions regarding the process.
In the past it was necessary to display both graphic images and alpha numeric images (alpha images) upon the display terminal. In previous process control systems such, as the TDC 3000 manufactured and sold by Honeywell Inc., alpha numeric data (or alpha data) was stored in one memory while graphics data was stored in another memory. Three possible display structures communicate the desired message to the display. The first display structure provides for the alpha foreground to be superimposed upon the alpha background, which was then superimposed upon the graphics image. The second possible display structure has the alpha foreground, superimposed upon the graphic image, which was then superimposed on the alpha background. The third and final possible display structure has the graphic image, superimposed on the alpha foreground, which was then superimposed upon the alpha background. These three possible display structures will be discussed in further detail later the present application.
A more recent concern in the field of process control systems is the ability for external systems to communicate with the process control system. Current process control systems such as the TDC 3000, previously mentioned, all provide a closed architecture allowing communication between only those devices which are part of the process control system. More recently it has been desired that an open architecture be devised thereby allowing external computer systems to access data and information from the process control system. This would allow connections between such systems as a VAX computer system manufactured by Digital Electronics Corp., Maynard, Massachusetts or an IBM System manufactured by International Business Machines. Many other computer systems exist and the afore-mention open architecture would allow virtually all of these systems to communicate with the process control system.
X-windows protocol provides for a network based, hierarchial windowing system which allows for high speed graphics. (X-windows was developed by the Massachusetts Institute of Technology and is a well known window system to those skilled in the art.) The X-windows protocol provides the ability to design an open architecture whereby current process control systems can be connected to outside computer systems via standard networks (e.g., DECnet manufactured by the Digital Electronics Corporation of Maynard, Massachusetts). X-windows protocol works as a bit map graphic system which allows high speed and high definition graphics.
Within the windowing system there is a server which controls each display device and a plurality of input devices (in the present embodiment referred to as an X-server). The display device is typically a Cathode Rate Tube (CRT) and the input device typically consists of key boards and/or pointing devices including a mouse, roller ball pointer, or a touch screen. Devices which are connected to the server and which attempt to communicate with the server are referred to as clients. Clients request certain actions of the server and the server may or may not carry out these requests. These actions include displaying information, reading information from the input devices, or moving things on the CRT.
As previously mentioned alpha data and graphic data are typically separated in process control systems. These two types of data are stored in separate memory devices and require proper blending to be properly displayed on the CRT. When a client requests that graphic and alpha numeric data be blended within the windowing system, the system cannot efficiently handle this blending.
When X-windows is called upon to blend two images together, the system will perform a Logical "OR" on the two images. This creates an unknown and undesirable result. To achieve the correct results in X-windows requires fairly complex logic operations. Seven bit wise logic operations are required to achieve the proper result. Execution of these operations causes this blending to be both slow and tedious.